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452 XIII. PROJECTION AND PROJECTORS
arc projector is required. Properly designed arc projectors are available that can deliver over 1000 lumens, and are suitable for a screen of approximately 10 ft. in width.
Projection Objective Lenses. The subject of lenses is an extensive one that usually requires many words and many figures if it is covered thoroughly. As the subject will be discussed here, no attempt will be made to cover lens design per se, the practical considerations concerning lenses available on the open market are more useful to most who will read this book.
There are numerous properties of a lens that should be checked by a purchaser. For all properties except resolving power there is no simple accepted standard method of testing. Fortunately, resolving power is the one property that needs most careful checking. The following are the other properties :
Chromatid Aberrations. This may be detected by the presence of a colored haze visible in the details over the whole field. Fortunately this is not a common defect.
Lateral Color. This is detected by the presence of one-sided color fringes appearing in the outer parts of the field and disappearing completely in the center.
Distortion. This is detected by straight lines in the outer part of the field appearing as curved lines on the screen. The straight sides of the picture gate of the projector itself make a good test.
Haze. This is detected by the presence of a misty haze of light covering the image without seriously interfering with the resolution. This is evidence of a large amount of spherical aberraton ; lenses showng this defect should not be used.
The tests for all these is the projection of a good reference film. If any one of the defects as described is apparent to a person viewing the screen, the lens should be rejected.
Most 16-mm projection objective lenses — especially those of 2-in. focal length and of //1.6 relative aperture — have a decidedly curved field, and the amount of curvature varies markedly among different manufacturers and among different lens types. Unfortunately, the manufacturers that make the most desirable machines do not ordinarily supply the best lenses. Resolving power measurements may be used as a criterion of the detailrendering quality of a lens and of its flatness of field in a manner similar to the resolving power measurements of film. To obtain data for comparison, it is necessary to decide upon a specific procedure for testing, since the setting of the lens along the optical axis for best focus at the center of the picture is very often different from its setting for best focus at the corners. Ordinarily, it is desirable to focus for best sharpness at